Radome nose for a missile, and method of making same



Aug. 22, 1967 P. B. WILFORD 3,336,873

RADOME NOSE FOR A MISSILE, AND METHOD OF MAKING SAME Filed June 28, 1965 MP jun- 1 FIG. I.

28 W P F W JL' L U &

J INVENTOR. 42 2; PAUL B. WILFORD FIG. 3 BY V- C. MULLER ATTORNEY.

United States Patent ()fllice 3 ,336,873 Patented Aug. 22, 1967 3,336,873 RADOME NOSE FOR A MISSILE, AND METHOD OF MAKING SAME Paul B. Wilford, Long Beach, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed June 28, 1965, Ser. No. 467,795 2 Claims. (Cl. 102-105) ABSTRACT OF THE DISCLOSURE The method of making a unitary missile nose shell of the type capped by a ceramic cap comprising the steps of: (1) spraying the ceramic cap as a deposit on the cavity of a female die element; (2) placing a preform of cloth impregnated with only incompletely thermoset impregnation agent on a mandrel die element and pressing the mandrel die element and female die element together under a force of approximately 500 p.s.i. while simultaneously applying heat to completely thermoset the impregnation agent; and (3) withdrawing the mandrel die element from the cavity of the female die element to pick up the deposited ceramic cap therefrom.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a novel missile nose cone article of manufacture, and to a method of making same. The nose cone has particular utility in connection with a missile that homes toward a source of radio frequency (R.F.) energy, where the nose cone houses an antenna system which is directionally sensitive to sources of RF. energy both ahead of the missile and in directions lateral to the missile heading.-

Many modern missiles are transported to the vicinity of a target by an aircraft in external mounting arrangement which exposes them to the airstream and the atmospheric environment. Moreover, the missiles are exposed to these airstream effects for long periods of time before they are fired, as the result of the aggregation of repeated routine armed missions in which the missile is not expended. As the speeds at which these aircrafts fly have increased, a serious problem of erosion of the outer skin has developed in instances where the aircraft must fly through raindrops, as for example, in geographic localities having heavy seasonal rains.

The problem is further aggravated where the nose cone encloses directional antenna apparatus for homing toward a source of RF. energy. In these instances, the nose cone must admit R.F. energy from both the direction ahead of the missile and the directions lateral to the missile. A very important requirement for this application is that the material, of which the nose cone is made, exhibits low diffraction type distortion effects. This desired quality is otherwise known as having a low dielectric loss-tangent coefl'lcient, or simply as having good window qualities for the admission of RF. energy. Obviously, freedom from defects and irregularities are also very important.

In an attempt to solve this problem, a variety of construction materials known to have good R.F. energy window qualities were tried, such as silicone, chopped fiber, epoxy woven sox, alumina, ribbon wound fiber, fiberglass, and fused silica. However, all showed unacceptable erosion upon high speed transport through rain conditions.

Accordingly, an object of the invention is to provide a novel missile nose cone article of manufacture which provides good mechanical resistance to erosive effects of high speed flight through rain, and also provides good R.F. energy window characteristics.

Another object is to provide a novel method for making an erosion resistant and RF. window missile nose cone, which method is relatively inexpensive and results in a very high percentage of the total production being free of imperfections and irregulaties.

Other objects, advantages and novel features of the in vention will become apparent from the following detailed description of the invention when considered in conjunction with the accompany drawings wherein:

FIG. 1 is a partial section view of a mold, which contains a missile nose cone unit, also shown in partially sectioned form, as the latter nose cone unit would appear during molding and prior to opening the mold;

FIG. 2 is a section of a portion of the mold of FIG. 1, in reduced scale, and shown receiving a sprayed deposit of ceramic material, the thickness of the deposit being exaggerated; and

FIG. 3 diagrammatically shows a nose cone unit employed in connection with a diagrammatic embodiment of anti-radar missile.

Referring now to the drawing, and in particular to FIG. 1, a cylindrical matched die mold assembly 10*, which is utilized in making the article of manufacture of the present invention, comprises a two-part female die 12, and a male die or mandrel 14. Female die 12 is formed from separable upper and lower die portions 12a and 12b, which are adapted for accurate vertical alignment in their assembled condition by a suitable dowel and guide hole arrangement 16. In addition to the mold dies, FIG. 1 also shows a nose cone unit 18 in pressed relationship between the mandrel and female die, as same would appear during molding and prior to opening of the die after molding. Nose cone unit 18 consists of a shell wall20, made of laminated glass cloth impregnated with a resin, and a nose cap insert 22 of ceramic material.

Female die 12 is formed from upper and lower die portions 12a and 12b, which together define the composite cavity 24 of the female die unit 12. The cavity surface 24b, of lower die portion 12b has a slightly frosted hard chrome finish to enhance ceramic cap separation, as will be presently understood. The shape of the exterior surface 26 of the mandrel has a shape which is complementary to that of cavity 24. A plurality of electric heating band units 28, each containing a resistance type heating wire 28a, are disposed about the mold assembly.

The method of making nose cone unit 18 will now be described. Lower female die portion 12b is removed from assembly with the upper portion. Then the cavity surface 24b of the lower section 12b is coated with a shear separation agent, consisting of a coating to which ceramic material may adhere, but which has low shear strength. Suitable separation agents include solutions of sodium chloride and solutions of borax. The nose cap insert 22 is then formed and shaped by building same up as a sprayed deposit of fine particles of aluminum oxide in their plastic or semi-molten state. This is done by means of conventional spray gun apparatus 30, FIG. 2,

of the type which heats the fine particles of ceramic maartisan techniques are employed in forming nose cap insert 22 in a manner to provide a hardness of 8 rnhos along the outer surface 34 of the insert. Also, the porosity of the deposit is gradually increased with increase of the built-up thickness of the insert in the direction away from the cavity face, so that the inner surface 36 of the insert is appreciably more porous than its outer surface 34. It has been found that the desired increase of porosity with thickness may be obtained as the inherent result of conventional building up of the deposit with a fixed setting of spray pressure and fixed size of particle supplied to the spray gun. For example, highly successful results have been achieved utilizing a type of commercially available spray gun apparatus known as a plasma gun, and employing a charge of granulated aluminum oxide having ninety-five percent (95%), by weight, of its particles in the range of sizes between 50 and 75 microns in their largest dimensions, and the remainder under microns in size. Spray gun apparatuses using rod stock of aluminum oxide also available, and may be employed as an alternative to apparatus which receives the aluminum oxide as a powder. After the deposit is formed, lower section 1219 is replaced in assembly with upper section 12a, and then the assembled die unit 12 is placed in a vertical hydraulic press (not shown), in which has also been placed the mandrel element 14.

The purpose of forming female die unit 12, as a twoiece element is now apparent. The part line 3'8, between the two die portions is adjacent to the circumferential locus that corresponds to the desired location of the rear marginal edge of ceramic cap insert 22. This permits access to the portion of the female die cavity to be sprayed.

The shell wall 20 of the nose cone unit 18 is first shaped and formed as a preform by a conventional low pressure laminating process. This preform is made of stage B, or semipolymerized epoxy resin impregnated glass cloth. The preform, which is still in its stage B condition, is slipped over mandrel 14. After a so-called debulking operation, in which the dies are heated and the press is manipulated to squeeze out excess resin, the pressure of the press is increased to approximately 500 p.s.i., and the die cavity temperature elevated to 325 F.- 350 F. The exterior surface of the stage B sheet material of the preform is initially molten and highly adhesive. Under the pressure of the press some of the resin material penetrates and saturates the relatively porous inner surface 36 of the nose cap insert 22. The mold is held at this temperature and pressure for the period of time necessary to polymerize and cure the resin, of the order of 20-30 minutes. This'forms an exceedingly high strength and imperfection and irregularity free bonded joint between shell wall 20 and cap insert 22. The strength of this joint is enhanced because of the penetration of the resin into the relatively porous inner surface 36 of the ceramic cap before curing. When, after curing, the mold is cracked or opened, the shell wall 20 remains adhered to mandrel element 14 with the cap insert 22 integrally bonded thereto. Thus the cap insert 22 is effectively picked up from die cavity 24b by the heat and pressure process which bonded the latent adhesive exterior surface of the preform to the insert. The nose cone unit 18 is removed from the mandrel and its outer face is polished by conventional contour grinding apparatus to insure good supersonic aerodynamic qualities. This removes any hairline irregularity appearing where the part line 38 of the two-piece female die was located, and also removes any slight discontinuity at the rear edge of the ceramic cap insert 22.

As mentioned in the introductory remarks, one particular adaptation of the present invention is in the field of a missile having a guidance system for homing toward a source of RF. energy, and which is to be externally carried by high speed aircraft. FIG. 3 diagrammatically illustrates the use of a nose cone unit 18' in such a missile 40 for homing against an enemy radar transmitter. The nose cone 18 houses directional antenna apparatus 42 having a plurality of individual antenna axes equiangularly displaced from missile axis. The signals from the displaced antennas are then fed to a mixing point 44 of the guidance system where they are differentially com- \bined to generate an error signal in accordance with the misalignment of the radar transmitter relative the missile bore sight axis. Reference is made to column 5, line 72 to column 6, line 39 of U.S. Patent No. 3,119,029 to Duane J. Russell for a generalized description of a missile of this type.

An important feature of nose cone unit 18 is that it meets both the mechanical strength and the RF. energy transmission requirements for such an application. The hardness of 8 mhos along the outer surface of ceramic cap 22 is roughly equivalent to that of Carborundum, and is sufficient to withstand the erosive effects ofhigh speed flight through raindrops for long aggregative periods of time encountered when the missile is carried by aircraft which fly repeated armed patrol missions in geographic localities having heavy rain conditions. The bonded construction of aluminum oxide over epoxy resin impregnated glass cloth has been found to provide highly desirable R.F. energy window qualities for the portion of the R.F. energy spectrum employed by modern radar stations.

In an alternative form of invention, the shell wall 20 of nose cone unit 18 may be constructed of phenolic resin impregnated glass cloth, in essentially the same manner as has been described. This results in a nose cone like unit 18, which does not have quite as excellent R.F. energy window properties, but which nevertheless is adequate for most uses. Where mechanical erosion resistance alone is required, i.e., without need for RF. energy window qualities, a variety of ceramic and thermosettable plastic construction materials could be employed with the method of this invention. Also, the heat resistance qualities of Ce ramic cap insert 22 of nose cone unit 18 makes it suitable for use with missiles which fly at the faster ultrasonic speeds at which aerodynamic heating problems occur at the nose tip.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method of making a ceramic capped nose cone shell for a missile in a matched die set for high pressure molding of a unitary missile nose shell, said die set consisting of mandrel die element having approximately the shape of the missile nose and female die element havinfg a conformal cavity, said method comprising the steps 0 (a) spraying fine particles of a ceramic material which are heated to their plastic state, onto the cavity surface of the female die element of the matched die set to form a continuous deposit of the desired shape and dimensions for the ceramic cap, said cavity surface being coated with a separation agent prior to spraying,

(b) forming and shaping a nose cone shell preform of a laminated cloth material impregnated with a thermosettable plastic impregnation and bonding agent, in which said impregnation and bonding agent is incompletely thermoset, and placing the preform over the mandrel die element of the matched die set, said impregnation and bonding agent being of a type provlding a latent adhesive surface along the exterior of the nose cone shell preform, said latent adhesive surface becoming active in response to heat,

(c) compressing the matched die set under applied compressive force of order of 500 pounds per square inch with said latent adhesive surface of the nose cone shell preform in contact with the exposed surface of the deposit of ceramic material, and heatmg same to a predetermined temperature for a predetermined period of time to bond and cure the preform and the deposited ceramic, and

(d) withdrawing the mandrel die element from the cavity of the female die element to separate the outwardly facing surface of the ceramic cap and the surface of the cavity.

2. The method in accordance with claim 1, said ceramic capped plastic nose cone shell being for a missile of a type which homes toward a radar transmitter, said shell having predetermined desired radio frequency (R.F.) energy Window characteristics for admitting R.F. energy from ahead of the missile and from lateral directions relative to the missile with three dimensional sensitivity characteristics symmetrically balanced about the missile axis, and in which;

(d) said ceramic material is aluminum oxide (A1 0 and substantially all of the particles deposited on the cavity surface are in the range of size between 50 and 75 microns in their langest particle dimension, said spraying being performed in a manner to pro duce an exterior surface having a hardness coefficient in excess of a value of eight (8) on the mho scale of hardness,

utes.

References Cited UNITED STATES PATENTS 2,765,248 10/1956 Beech et al. 156-232 3,179,531 4/1965 Koubek 117-23 3,292,544 10/ 1966 Caldwell et al 102-925 BENJAMIN A. BORCHELT, Primary Examiner. ROBERT F. STAHL, Examiner. 

1. A METHOD OF MAKING A CERAMIC CAPPED NOSE CONE SHELL FOR A MISSLE IN A MATCHED DIE SET FOR HIGH PRESSURE MOLDING OF A UNITARY MISSILE NOSE SHELL, SAID DIE SET CONSISTING OF MANDREL DIE ELEMENT HAVING APPROXIMATELY THE SHAPE OF THE MISSILE NOSE AND FEMALE DIE ELEMENT HAVING A CONFORMAL CAVITY, SAID METHOD COMPRISING THE STEPS OF: (A) SPRAYING FINE PARTICLES OF A CERAMIC MATERIAL WHICH ARE HEATED TO THEIR PLASTIC STATE, ONTO THE CAVITY SURFACE OF THE FEMALE DIE ELEMENT OF THE MATCHED DIE SET TO FORM A CONTINUOUS DEPOSIT OF THE DESIRED SHAPE AND DIMENSIONS FOR THE CERAMIC CAP, SAID CAVITY SURFACE BEING COATED WITH A SEPARATION AGENT PRIOR TO SPRAYING, (B) FORMING AND SHAPING A NOSE CONE SHELL PREFORM OF A LAMINATED CLOTH MATERIAL IMPREGNATED WITH A THERMOSETTABLE PLASTIC IMPREGNATION AND BONDING AGENT, IN WHICH SAID IMPREGNATION AND BONDING AGENT IS INCOMPLETELY THERMOSET, AND PLACING THE PREFORM OVER THE MANDREL DIE ELEMENT OF THE MATCHED DIE SET, SAID IMPREGNATION AND BONDING AGENT BEING OF A TYPE PROVIDING A LATENT ADHESIVE SURFACE ALONG THE EXTERIOR OF THE NOSE CONE SHELL PREFORM, SAID LATENT ADHESIVE SURFACE BECOMING ACTIVE IN RESPONSE TO HEAT, (C) COMPRESSING THE MATCHED DIE SET UNDER APPLIED COMPRESSIVE FORCE OF ORDER OF 500 POUNDS PER SQUARE INCH WITH SAID LATENT ADHESIVE SURFACE OF THE NOSE CONE SHELL PREFORM IN CONTACT WITH THE EXPOSED SURFACE OF THE DEPOSIT OF CERAMIC MATERIAL, AND HEATING SAME TO A PREDETERMINED TEMPERATURE FOR A PREDETERMINED PERIOD OF TIME TO BOND AND CURE THE PREFORM AND THE DEPOSITED CERAMIC, AND (D) WITHDRAWING THE MANDREL DIE ELEMENT FROM THE CAVITY OF THE FEMALE DIE ELEMENT TO SEPARATE THE OUTWARDLY FACING SURFACE OF THE CERAMIC CAP AND THE SURFACE OF THE CAVITY. 